1. Field of the Invention
The present invention is concerned with the variable control of an electrochromic window. The work window is being used here as a generic term to mean an electrochemical structure which can be reversibly altered between the transparent colourless state and the transparent coloured state.
2. Description of the Prior Art
The current state of electrochromic window technology is that of a structure made of two active electrodes, which together form an electrochemical cell, this cell having other transparent conducting layers, and a transparent electrolyte arranged so that the overall structure changes colour density on the passage of an electric current. This colour change can be completely reversed on inversion of the current.
One such electrochromic window structure is a complex comprising two sheets of glass which act as the transparent external walls, coated on the inside with thin transparent conductor layers, and using two further suitable thin films as active electrodes for the system with a suitable electrolyte as the element for transmission of active ionic species. One of the two active electrodes is a thin film of tungsten oxide, whilst the other active electrode comprises a solid solution of lithium in a thin film of vanadium oxide (or a mixture of vanadium oxide and molybdenum oxide). The electrolyte used is a polymer made by dissolving lithium perchlorate in a suitable solution of propylene carbonate and polymethyl methacrylate.
The application of a potential difference across this window structure, with the negative potential on the tungsten trioxide, will cause a current to flow and the window to become coloured. If the current is reversed the window will become lighter in colour until finally becoming virtually colourless.
Electrochromic windows of a size suitable for automotive and architectural applications have been described in European patent application No. 0608203.